Outrage, partly prompted by the Daily Mail, caused the double jeopardy rule to be changed

Two of Stephen Lawrence's killers Gary Dobson and David Norris have been sentenced to minimum life terms "at her Her Majesty's Pleasure" of 15 years 2 months and and 14 years 3 months respectively.

There has been surprise, from the Daily Mail among others, that Dobson and Norris, now in their mid-30s, were sentenced as juveniles. Curiously, they have also been sentenced under historic law dating back to around 1993, which means they could not be sentenced under harsh new guidance for racially aggravated crimes.

This may all sound a bit strange, but as readers of this blog will know, the sentencing of criminals convicted in "cold cases" which have heated up can be much more complicated than if the crime happened a short while before trial. This may upset Daily Mail readers, but the reason is partly the European Convention on Human Rights. As Alasdair Henderson posted last month, article 7 prohibits retrospective punishment, that is punishment using law which was not applicable at the time of the crime:

(1) No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence under national or international law at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the criminal offence was committed.

(2) This article shall not prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according to the general principles of law recognised by civilised norms.

Article 7 must be read together with the long-standing principle in criminal law that sentencing is passed as if the crime had happened a short time ago; this only becomes controversial in rare cases when a case has been reopened after many years.

Article 7 (1) will only be infringed if a sentence is imposed on a defendant which constitutes a heavier penalty than that which could have been imposed on the defendant under the law in force at the time that his offence was committed.

So the discretion of the judge sentencing Dobson and Norris only expands to the maximum sentence which could have been handed down to them if the matter had been tried shortly after the crime took place. At that time, in 1993 and early 1994, they were juveniles so must be sentenced as if they still were.

This means, thankfully, the judge does not have to imagine what the killers would have received at the time of the crime. But this is still a very complex task. As any criminal lawyer knows, a glut of sentencing legislation and guidance has been passed since 1993, when the murder took place, all of which must be ignored.

However, help is at hand from the court of appeal which, in the November 2011 case of H, R. v (Rev 2) [2011] EWCA Crim 2753 provided the following guidance for sentencing in "cold cases":

(a) Sentence will be imposed at the date of the sentencing hearing, on the basis of the legislative provisions then current, and by measured reference to any definitive sentencing guidelines relevant to the situation revealed by the established facts.

(b) Although sentence must be limited to the maximum sentence at the date when the offence was committed, it is wholly unrealistic to attempt an assessment of sentence by seeking to identify in 2011 what the sentence for the individual offence was likely to have been if the offence had come to light at or shortly after the date when it was committed. Similarly, if maximum sentences have been reduced, as in some instances, for example theft, they have, the more severe attitude to the offence in earlier years, even if it could be established, should not apply.

(c) As always, the particular circumstances in which the offence was committed and its seriousness must be the main focus. Due allowance for the passage of time may be appropriate. The date may have a considerable bearing on the offender's culpability. If, for example, the offender was very young and immature at the time when the case was committed, that remains a continuing feature of the sentencing decision. Similarly if the allegations had come to light many years earlier, and when confronted with them, the defendant had admitted them, but for whatever reason, the complaint had not been drawn to the attention of, or investigated by, the police, or had been investigated and not then pursued to trial, these too would be relevant features.

(d) In some cases it may be safe to assume that the fact that, notwithstanding the passage of years, the victim has chosen spontaneously to report what happened to him or her in his or her childhood or younger years would be an indication of continuing inner turmoil. However the circumstances in which the facts come to light varies, and careful judgment of the harm done to the victim is always a critical feature of the sentencing decision. Simultaneously, equal care needs to be taken to assess the true extent of the defendant's criminality by reference to what he actually did and the circumstances in which he did it.

(e) The passing of the years may demonstrate aggravating features if, for example, the defendant has continued to commit sexual crime or he represents a continuing risk to the public. On the other hand, mitigation may be found in an unblemished life over the years since the offences were committed, particularly if accompanied by evidence of positive good character.

(g) Early admissions and a guilty plea are of particular importance in historic cases. Just because they relate to facts which are long passed, the defendant will inevitably be tempted to lie his way out of the allegations. It is greatly to his credit if he makes early admissions. Even more powerful mitigation is available to the offender who out of a sense of guilt and remorse reports himself to the authorities. Considerations like these provide the victim with vindication, often a feature of great importance to them.

An unenviable task for the judge, and no doubt one which will lead to criticism of the sentencing regime. But, given the clear words of article 7 ECHR, it would be difficult for the law to be changed to allow for retrospective punishment which is more severe than it would have been at the time.

In any case, it is important to remember that it was only because of the outrage – prompted in part by the famous Daily Mail "murderers" headline - over the initial failed private prosecution that the law on double jeopardy (trying people for the same crime twice) was changed to allow Dobson and Norris to be tried again.

The change in the double jeopardy rules, and more significantly the advances in the use of DNA to solve old crimes, means that an increasing number of judges will have to set their minds to the strange and artificial task of sentencing according to old law. But the human rights restrictions behind this exercise remain clear, and are unlikely to change any time soon.

• This article was amended on 4 January 2012 to correct a reference that gave 1993 and early 2004 as a period when Gary Dobson and David Norris were still juveniles. This has been changed to 1993 and early 1994.

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